High performance cooling element

ABSTRACT

The present invention relates to an external cooling system for a molten film tube produced by a blown film tubular extrusion process, comprised of a divergent cooling element with a divergent cooling interface containing a cooling gas deflector spaced adjacent to the molten film tube and providing an expelled cooling gas (i) in a path opposing the flow of the molten film tube toward a first exit gap and (ii) in a path with the flow of the molten film tube toward a second exit gap. A minimum gap between the divergent cooling interface and the molten film tube occurs at the first exit gap and/or the second exit gap. Advantageously, the divergent cooling interface is provided with one or more compound angles to maximize stability and cooling efficiency. Additionally, multiple cooling elements can preferably be arranged in a stackable configuration to achieve higher throughput rates. Operation is characterized by improved film holding forces without the presence of high noise levels or detrimental vibration, flutter, and drag. Additionally, employing simplified single air delivery channels, and a stackable design, significantly reduces complexity and manufacturing costs.

BACKGROUND OF THE INVENTION

Field of the Invention

The present disclosure relates to a method and apparatus for cooling.The present disclosure relates more particularly to a method andapparatus for high performance cooling.

Description of Related Art

Various methods to manufacture thermoplastic blown films are well knownin the plastics art, and typically involve forming a continuous,vertically oriented, seamless, annular plastic film commonly referred toas the “tube” or “bubble”. Thermoplastic material is melted and pumpedby an extruder through a blown film die (die), exiting as an annularflow of a molten film, continuously drawn upward by a pair of drivensqueeze rollers. Gas is typically manually injected through the die tothe interior of the exiting annular flow of molten film. The drivensqueeze rollers act to prevent gas from escaping, trapping the injectedgas inside, forming a molten film tube which is inflated by the injectedgas until at the desired size and the die is sealed off to preventinflation gas from escaping. The molten film tube is pulled upward bythe driven squeeze rollers, flowing generally upward from the diethrough a cooling system, where it stretches, expands, and cools aroundthe now trapped column of injected gas until it solidifies at a frostline into a solidified film tube. The solidified film tube passesthrough various stabilizers and enters a flattening device, whichconverts the tube into a flattened double thickness thermoplastic sheetof film known as “lay-flat”. The lay-flat passes through the drivensqueeze rollers, and is conveyed to downstream converting equipment suchas winders and bag making machines for further processing.

To remain competitive, manufacturers of blown film must maximizethroughput rate and quality, however cooling system performance is asignificant limiting factor. The weight of thermoplastic that isextruded per unit time divided by the circumference of the die exit,provides a commonly used measure of throughput performance, and isexpressed in units of PPH/Inch, Pounds Per Hour per Inch of die exitcircumference. Many different cooling systems have been developed andemployed, both external and internal to the tube, and to varying degreesthese systems have achieved commercial success.

Blown film cooling systems provide a flow of cooling gas typicallyexternal, but in many cases also internal to the molten film tube.Cooling systems are designed using well known Bernoulli and Coand{hacekover (a)} principles, and in many cases, apply the cooling gas to flowgenerally along the surface of the molten film tube to create holdingforces on the molten film tube, providing for both stability and coolingof the molten film tube. Excessive holding forces, if present, can causevibration, flutter, and high noise levels in the process, and can pullthe molten film tube into undesirable contact with the cooling element,creating drag and causing marks and instability in the molten film tube.In other cases, cooling gas is instead applied generally against thesurface of the molten film tube, typically creating turbulent coolingwith repelling forces, requiring a separate means to stabilize themolten film tube.

External cooling systems, generally provide the primary means forstabilization and cooling of the molten film tube, are generally easy tooperate and used on most blown film extrusion processes. Externalcooling systems provide a flow of cooling gas along the outside surfaceof the molten film tube that typically generates holding forces whilecooling the molten film tube, until the cooling gas dissipates into thesurrounding atmosphere. Less typically, cooling gas is aimed generallyinward generating repelling forces while cooling the molten film tube,undesirably requiring a separate means to hold and stabilize the moltenfilm tube.

Present art external cooling systems are made up of various types ofcooling elements. The earliest cooling element, known as a “Single Flowair ring”, still in common use today, applies a single flow of coolinggas around the molten film tube. Single Flow cooling elements typicallyproduce good film quality, but at lower throughput rates. Additionalflows of cooling gas have been added to cooling elements over time tocreate various multiple flow designs, such as “Dual Flow”, “Triple Flow”or “Quad Flow” designs, and some external cooling systems pair coolingelements into various configurations, depending on the application, toform what is generically known as a “Tandem” air ring. External coolingsystems are typically fixed in place, but can be made adjustable inheight above the die to allow extending the cooled surface area alongthe molten film tube, producing higher throughput, but also resulting ingreater unsupported surface area between the cooling element and die,which is the hottest and weakest portion of the molten film tube, whichcan lead to degraded stability, making it more difficult to operate andpotentially leading to a narrower range of film sizes.

In contrast, internal cooling systems typically do not provide primarystabilization, and are selectively used typically to generate additionalthroughput beyond the capability of an external cooling system. Internalcooling systems replace manual gas injection and inflate the molten filmtube with a flow of an internal supply gas that enters through the die.Although some recent high throughput internal cooling systems applycooling gas to create holding forces, more typically cooling gas isdirected against the inside surface of the molten film tube, acting togenerally repel and cool the inside surface of the molten film tube. Theflow of internal supply gas is trapped inside the bubble and cannotdissipate into the atmosphere, therefore complex control systems areused to balance a flow of internal exhaust gas that exits through thedie to maintain a constant bubble size as is well known and understoodby those skilled in the art. Internal cooling systems can be difficultor even impossible to use depending on such factors as operator skill,thermoplastic material properties, and the physical size and design ofthe associated die.

It is highly desired to overcome the drawbacks of prior artthermoplastic cooling systems and provide a cooling system thatsignificantly increases throughput rate, maximizes aerodynamic holdingforces, allows relatively large unsupported regions of the molten tubewith good stability, produces a wide range of film sizes, prevents dragon the molten film surface, minimizes turbulence, vibration and flutter,does not produce high sound power levels, and is simple and easy tocontrol.

SUMMARY OF THE INVENTION

In view of the foregoing, it is an object of the present disclosure toprovide a method and apparatus for cooling.

A first exemplary embodiment of the present disclosure presents anapparatus for cooling. The apparatus includes at least one divergentcooling element for receiving a flow of a molten film tube, the at leastone divergent cooling element including a divergent cooling interfaceoperable for expelling a cooling gas (i) in a path opposing the flow ofthe molten film tube toward a first exit gap and (ii) in a path with theflow of the molten film tube toward a second exit gap, wherein at leastone of the first exit gap and the second exit gap define a minimum gapbetween the divergent cooling interface and the flow of the molten filmtube.

A second exemplary embodiment includes wherein the divergent coolinginterface includes a cooling gas deflector for directing expelledcooling gas along the path opposing the flow of the molten film tube andalong the path with the flow of the molten film tube.

A third exemplary embodiment includes wherein a portion of the divergentcooling interface in the path opposing the flow of the molten film tubeforms one or more compound angles, and wherein a portion of thedivergent cooling interface in the path with the flow of the molten filmforms one or more compound angles.

A fourth exemplary embodiment of the present disclosure presents amethod for cooling. The method includes receiving, by at least onedivergent cooling element, a flow of a molten film tube. The methodfurther includes cooling, by the at least one divergent cooling element,the flow of the molten film tube, wherein the at least one divergentcooling element comprises a divergent cooling interface operable forexpelling a cooling gas (i) in a path opposing the flow of the moltenfilm tube toward a first exit gap and (ii) in a path with the flow ofthe molten film tube toward a second exit gap, wherein at least one ofthe first exit gap and the second exit gap define a minimum gap betweenthe divergent cooling interface and the flow of the molten film tube.

The following will describe embodiments of the present invention, but itshould be appreciated that the present invention is not limited to thedescribed embodiments and various modifications of the invention arepossible without departing from the basic principles. The scope of thepresent disclosure is therefore to be determined solely by the appendedclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional view of a device suitable for use inpracticing exemplary embodiments of this disclosure.

FIG. 2 is a close-up cross sectional view of an exemplary coolingelement suitable for use in practicing exemplary embodiments of thisdisclosure.

FIG. 3 is a close-up cross sectional view of an alternative exemplarycooling element suitable for use in practicing exemplary embodiments ofthis disclosure.

FIG. 4 is a cross sectional view of an alternative device suitable foruse in practicing exemplary embodiments of this disclosure.

FIG. 5 is a logic flow diagram in accordance with a method and apparatusfor performing exemplary embodiments of this disclosure.

DETAILED DESCRIPTION OF THE INVENTION

Exemplary embodiments of the present disclosure relate to a highperformance cooling system for the blown film tubular extrusion processproviding increased throughput rate at high quality. Embodiments of thehigh performance cooling system include one or more cooling elements,capable of being stacked to achieve higher throughput, wherein at leastone of the one or more cooling elements is a divergent cooling elementincorporating a cooling gas deflector and a pair of opposed coolingmembers having respective opposed air foil surfaces (surfaces). Thesurfaces and cooling gas deflector form a divergent cooling interfacethat directs cooling gas to flow in opposite directions, creatingsuction forces. These suction forces stabilize and hold the molten filmtube (melt) in cooling proximity with the divergent cooling element.Oppositely directed cooling gas flows are expelled between the divergentcooling interface and the melt (i) in a path opposing the flow of themolten film tube along a first surface toward a first air foil exit tipto form a first exit gap with the melt and (ii) in a path with the flowof the molten film tube along a second surface toward a second air foilexit tip to form a second exit gap with the melt.

The divergent cooling interface includes air foil exit tips that areangled inward toward the surface of the molten film tube, and extendcloser to the molten film tube than any other portion of the divergentcooling element. All portions of the divergent cooling interface arerecessed within the air foil exit tips away from the molten film tube,preventing hang-up inducing drag. The angled in air foil exit tips actto compress the cooling gas stream, providing a cushioning effect thatdampen vibrations and flutter in the molten film tube, and eliminateexcessive sound power levels. Additionally, the angled in air foil exittips act similar to a venturi, to accelerate the cooling gas streamsflowing along the molten film tube to a higher velocity where thecooling gas streams emerge from the influence of associated coolingmembers at the air foil exit tips and flow along the molten film tube.This higher velocity cooling gas flow translates into higher coolingefficiency and throughput.

Further, one or more compound angles are preferentially employed alongthe air foil surfaces. Larger surface angles (up to about 45 degreesfrom parallel to the molten film surface) provide greater compression ofthe cooling gas, which desirably improves cooling efficiency, butunfortunately, also can cause an undesirable reduction in holding force.Use of compound angles wraps the cooling gas flow smoothly aroundcooling element air foil surfaces, which are advantageously arranged tochange from a larger to a smaller angle relative to the molten filmtube, in the direction of cooling gas flow, prior reaching the air foilexit tips. This approach allows larger initial cooling member air foilsurface angles which act to aggressively compress the cooling gasstreams for maximum cooling efficiency, followed by smaller surfaceangles, just prior and up to the air foil exit tips, which act toaccelerate the cooling gas streams, restoring holding force to amaximum. Divergent cooling elements with compound angles in the coolingmember air flow surfaces, exhibit very high cooling efficiency, maximumholding force, and excellent stability, without vibration, flutter orhigh sound power levels.

Advantageously, a simplified single air delivery channel is providedbetween first and second cooling members, feeding an inward radial flowof cooling gas arranged to impinge on an outer wall of a cooling gasdeflector, preventing direct cooling gas flow against the blown filmtube and separating the flow into oppositely directed first and secondannular cooling gas streams. The cooling gas deflector generallyarranged intermediate between the cooling members and the molten tubesuch that the inner wall of the cooling gas deflector is spaced furtherfrom the molten tube than the associated cooling member tips to preventdrag. Additional flows of cooling gas can be advantageously added, butare not required.

Embodiments of a divergent flow, high performance cooling element of thepresent invention include a simplified air delivery channel feeding oneor more compound angle cooling member air foil surface with an initial22.5 degree angle followed by a 7.5 degree air foil exit tip angle,provide an increased holding force, reduced vibration and flutterresulting in measured sound power levers 18 db lower (64 times less)than divergent cooling elements with a 0 degree air foil exit tip angle.Embodiments of the present disclosure include one or more compound anglecooling member air foil surface with an initial angle between 15 and 25degrees, followed by an exit tip angle between 5 and 15 degrees withexcellent stability, efficiency gains, increased holding force, reducedvibration and flutter. However, it should be appreciated thatembodiments include a cooling gas foil surface and an exit tip anglewith any combination of compound angles that aid in increasingthroughput, stability, and in reducing vibration and flutter.

FIG. 1 shows a cross sectional view of a typical blown film extrusionprocess employing a short stack cooling system with divergent coolingelements of the present invention. In FIG. 1-FIG. 4, all thin arrowsindicating a direction are for illustrative purposes only, labeled forexample as AF, and indicate a direction flow of a fluid (e.g. coolinggas). Further, Thick arrows indicating a direction are for illustrativepurposes only, labeled for example as MF, and indicate a direction flowof a plastic film material (e.g. molten film tube). Thermoplastic resinis introduced through feed hopper 2 into extruder 4 where the resin ismelted, mixed and pressurized. Molten resin is conveyed through meltpipe 6 into a die means 8 that forms it into an annular molten flow thatexits generally from the top surface of die means 8 as a molten filmtube 12.

Internal gas supply conduit 10 operably provides an internalcooling/inflating gas through die means 8 to the interior of molten filmtube 12 and solidified film tube 16. Internal gas exhaust conduit 9operably removes internal cooling/inflating gas through die means 8 asrequired to maintain a desired trapped tube volume of gas inside moltenfilm tube 12 and solidified film tube 16, further contained by niprollers 20. Gas flow through Internal gas supply conduit 10 and Internalgas exhaust conduit 9 are controlled by methods commonly understood bythose skilled in the art. Molten film tube 12 expands outwardly aroundthe trapped tube volume of gas and is drawn upwardly by nip rollers 20while being cooled to solidify at freeze line 14 forming solidified filmtube 16. Solidified film tube 16 is collapsed by flattening guides 18before passing through nip rollers 20 forming flattened film 22.Flattened film 22 is then conveyed to downstream equipment forconversion into usable products as desired.

Annular cooling elements 23, 24 a-d, and 26 are arranged coaxial withand in the direction of flow of molten film tube 12. Cooling elements23, 24 a-d, and 26, each supplied with cooling gas from a suitableexternal source, direct associated cooling gas alongside molten filmtube 12, generally in the same and/or opposite direction to the flow ofmolten film tube 12, acting to stabilize and cool molten film tube 12.

Upward cooling gas traveling generally in the direction of flow ofmolten film tube 12 from cooling elements 23 and 24 a-c, and downwardcooling gas traveling generally opposite the direction of flow of moltenfilm tube 12 from cooling elements 24 a-d flows directly into a cavity Caround molten film tube 12. Cavity C is contained and isolated from thesurrounding atmosphere by enclosure 28 with additional extents formed bythe portion of the cooling elements 23 and 24 a-d in contact with cavityC (cooling element cavity portion), and the portion of the molten filmtube 12 in contact with cavity C (molten film cavity portion). Coolinggas entering cavity C flows alongside and cools molten film tube 12, andexhausts between cooling elements 23 and 24 a-d to the surroundingatmosphere. Generally upwardly directed cooling gas from cooling element26 flows unrestricted, along molten film tube 12, directly influenced bythe surrounding atmosphere, while cooling and allowing for freeexpansion of molten film tube 12.

FIG. 2 shows a cross sectional view of the inner portion of one half ofa divergent cooling element, of the present invention. Each divergentcooling element (FIG. 1, 24 a-d) is provided with an interior air plenum38 of any suitable shape and size, that directs the associated suppliedcooling gas to flow generally radially inward through annular channel 40formed between cooling members 42 and 44. Annular channel 40 feedscooling gas toward cooling gas deflector 46, inwardly supported fromcooling members 42 and/or 44 using common, readily available fasteningmeans such as screws and washers, not shown. Cooling gas deflector 46prevents direct cooling gas flow against molten film tube 12 andseparates cooling gas flow into oppositely directed cooling gas streams48 and 50. Cooling gas stream 48 flows generally opposite the directionof the flow of molten film tube 12, between air foil surface 52 andmolten film tube 12. Cooling gas stream 50 flows generally in the samedirection of flow of molten film tube 12, between air foil surface 54and molten film tube 12.

Air foil surfaces 52 and 54 are annularly angled inward toward moltenfilm tube 12, in the direction of respective air flow just prior toterminating at air foil exit tips 56 and 58, where cooling gas streams48 and 50 leave the influence of air foil surfaces 52 and 54respectively and flow in cooling contact along the surface of moltenfilm tube 12. Compound angles are preferentially employed along air foilsurfaces 52 and 54 with larger angles relative to molten film tube 12located nearest to cooling gas deflector 46, and smaller angles locatedadjacent respective air foil exit tips 56 and 58. Importantly, noportion of the divergent cooling element having a divergent coolinginterface comprised of cooling gas deflector 46, air foil surfaces 52and 54, and air foil exit tips 56 and 58 is closer to the molten filmtube than either one or both of the air foil exit tips 56 and 58, toensure that no mechanical contact with molten film tube 12 can occur.

In FIG. 3, cooling gas deflector 46 is replaced by cooling gas deflector46 a located intermediate cooling members 42 and 44, forming a pair ofannular channels 40 a and 40 b. Cooling gas from interior air plenum 38generally flows radially inward, independently through annular channels40 a and 40 b, exiting the influence of cooling gas deflector 46 a asoppositely directed cooling gas streams 48 and 50, respectively. Coolinggas stream 48 flows generally opposite the direction of flow of moltenfilm tube 12, between air foil surface 52 and molten film tube 12.Cooling gas stream 50 flows generally in the same direction of flow ofmolten film tube 12, between air foil surface 54 and molten film tube12.

As described in FIG. 2, air foil surfaces 52 and 54 are annularly angledinward toward molten film tube 12, in the direction of respective airflow just prior to terminating at air foil exit tips 56 and 58, wherecooling gas streams 48 and 50 leave the influence of air foil surfaces52 and 54 respectively and flow in cooling contact along the surface ofmolten film tube 12. Compound angles are preferentially employed alongair foil surfaces 52 and 54 with larger angles relative to molten filmtube 12 located nearest to cooling gas deflector 46 a, and smallerangles located adjacent respective air foil exit tips 56 and 58.Importantly, no portion of the divergent cooling element having adivergent cooling interface comprised of cooling gas deflector 46 a, airfoil surfaces 52 and 54, and air foil exit tips 56 and 58 is closer tothe molten film tube than either one or both of the air foil exit tips56 and 58, to ensure that no mechanical contact with molten film tube 12can occur.

FIG. 4 depicts a cooling system employing high performance, divergentcooling elements of the present invention in a configuration similar toFIG. 1, but with the addition of a enclosure 28, variable exhaust device30, variable controller means 32, and flow buffer 34 with freelyswinging flapper 36 as described in co-pending application titledControlled Pressure Enclosure filed on Jan. 15, 2016 with first namedinventor Robert E. Cree, filed under attorney docket number100646.000004, the contents of which is hereby incorporated byreference. Cooling gas supply conduits 60 are also added, spacedgenerally inside and around the perimeter of cooling elements 23, 24 a,24 b and 24 c, forming a common supply of cooling gas. Cooling gassupply conduits 60 also act to space apart and locate concentric tomolten film tube 12 each of the associated cooling elements 23, 24 a, 24b and 24 c. Cooling element 24 d is advantageously shown supplied withcooling gas in common with cooling element 26, forming a highperformance triple flow air ring. Cooling element 26 is depicted withone single-flow of cooling gas, but can include more than one flow ofcooling gas, forming further high performance multiple-flow versions incombination with cooling element 24 d of the present invention, exitingto flow unrestricted, generally upward and along molten film tube 12,directly influenced by the surrounding atmosphere, while cooling andallowing for free expansion of molten film tube 12. Cooling element 26may also be omitted, allowing cooling gas exiting from the upper mostlocated high performance, divergent cooling element to either be locatedbelow frost line 14 and allow for free expansion or be located abovefrost line 14 and constrain the molten film tube 12.

The present invention is presented on an upward blown film extrusionprocess, but equally applies to horizontal or downward versions of theblown film extrusion process, without limit. Further, the presentinvention can be employed in a linear rather than annular configuration,and applies to collapsing frame stabilization as well as single sheetcast film prior art.

Referring to FIG. 5, presented is an exemplary logic flow diagram inaccordance with a method and apparatus for performing exemplaryembodiments of this disclosure. Block 502 presents receiving, by atleast one divergent cooling element, a flow of a molten film tube; andcooling, by the at least one divergent cooling element, the flow of themolten film tube, wherein the at least one divergent cooling elementcomprises a divergent cooling interface operable for expelling a coolinggas (i) in a path opposing the flow of the molten film tube toward afirst exit gap and (ii) in a path with the flow of the molten film tubetoward a second exit gap, wherein at least one of the first exit gap andthe second exit gap define a minimum gap between the divergent coolinginterface and the flow of the molten film tube. Block 504 relates towherein the at least one divergent cooling interface comprises a coolinggas deflector for directing expelled cooling gas along the path opposingthe flow of the molten film tube and along the path with the flow of themolten film tube.

Then block 506 indicates further comprising cooling by a second coolingelement stacked adjacent the at least one divergent cooling element.Block 508 specifies further comprising cooling the flow of the moltenfilm tube by at least one of a triple flow air ring and a multiple flowair ring. Block 510 then indicates wherein a space is defined betweenthe at least one divergent cooling element and the second coolingelement to allow gas exchange with a surrounding atmosphere. Block 512then states wherein a portion of the divergent cooling interfaceexpelling the cooling gas in the path opposing the flow of the moltenfilm tube forms compound angles, and wherein a portion of the divergentcooling interface expelling the cooling gas in the path with the flow ofthe molten film forms compound angles.

Block 514 relates to wherein the expelled cooling gas from the at leastone divergent cooling element sufficiently cools the molten film tube ata rate between 0.5 and 5 (pounds/hour)/(inch of die circumference).Finally block 516 then states wherein at least a portion of the coolinggas is received by at least one enclosure comprising a cavity forreceiving the cooling gas from the at least one divergent coolingelement, the at least one enclosure operable to maintain a predeterminedpressure differential between an inside surface and an outside surfaceof the flow of the molten film tube.

The logic flow diagram may be considered to illustrate the operation ofa method. The logic flow diagram may also be considered a specificmanner in which components of a device are configured to cause thatdevice to operate, whether such a device is a blown film tubularextrusion device, controlled pressure enclosure, or divergent coolingelement, or one or more components thereof.

Embodiments of the present invention has been described in detail withparticular reference to particular embodiments, but it will beunderstood that variations and modifications can be effected within thespirit and scope of the invention. The presently disclosed embodimentsare therefore considered in all respects to be illustrative and notrestrictive. The scope of the invention is indicated by the appendedclaims, and all changes that come within the meaning and range ofequivalents thereof are intended to be embraced therein.

1. An apparatus for cooling, the apparatus comprising: at least onedivergent cooling element for receiving a flow of a molten film tube,the at least one divergent cooling element comprising a divergentcooling interface operable for expelling a cooling gas (i) in a pathopposing the flow of the molten film tube toward a first exit gap and(ii) in a path with the flow of the molten film tube toward a secondexit gap, wherein at least one of the first exit gap and the second exitgap define a minimum gap between the divergent cooling interface and theflow of the molten film tube.
 2. The apparatus according to claim 1,wherein the divergent cooling interface comprises a cooling gasdeflector for directing expelled cooling gas along the path opposing theflow of the molten film tube and along the path with the flow of themolten film tube.
 3. The apparatus according to claim 2, the apparatusfurther comprising a second cooling element operably stacked adjacentthe at least one divergent cooling element.
 4. The apparatus accordingto claim 3, wherein a space is defined between the at least onedivergent cooling element and the second cooling element to allow gasexchange with a surrounding atmosphere.
 5. The apparatus according toclaim 3, the apparatus further comprising at least one of a triple flowair ring and a multiple flow air ring.
 6. The apparatus according toclaim 1, wherein a portion of the divergent cooling interface expellingthe cooling gas in the path opposing the flow of the molten film tubeforms compound angles, and wherein a portion of the divergent coolinginterface expelling the cooling gas in the path with the flow of themolten film forms compound angles.
 7. The apparatus according to claim1, wherein the expelled cooling gas from the at least one divergentcooling element sufficiently cools the molten film at a rate between 0.5and 5 (pounds/hour)/(inch of die circumference).
 8. The apparatusaccording to claim 1, the apparatus further comprising at least oneenclosure comprising a cavity for receiving at least a portion of thecooling gas from the at least one divergent cooling element, the atleast one enclosure operable to maintain a predetermined pressuredifferential between an inside surface and an outside surface of theflow of the molten film tube.
 9. A method for cooling, the methodcomprising: (a) receiving, by at least one divergent cooling element, aflow of a molten film tube; and (b) cooling, by the at least onedivergent cooling element, the flow of the molten film tube, wherein theat least one divergent cooling element comprises a divergent coolinginterface operable for expelling a cooling gas (i) in a path opposingthe flow of the molten film tube toward a first exit gap and (ii) in apath with the flow of the molten film tube toward a second exit gap,wherein at least one of the first exit gap and the second exit gapdefine a minimum gap between the divergent cooling interface and theflow of the molten film tube.
 10. The method according to claim 9,wherein the at least one divergent cooling interface comprises a coolinggas deflector for directing expelled cooling gas along the path opposingthe flow of the molten film tube and along the path with the flow of themolten film tube.
 11. The method according to claim 10, the methodfurther comprising cooling by a second cooling element stacked adjacentthe at least one divergent cooling element.
 12. The method according toclaim 10, the method further comprising cooling the flow of the moltenfilm tube by at least one of a triple flow air ring and a multiple flowair ring.
 13. The method according to claim 11, wherein a space isdefined between the at least one divergent cooling element and thesecond cooling element to allow gas exchange with a surroundingatmosphere.
 14. The method according to claim 9, wherein a portion ofthe divergent cooling interface expelling the cooling gas in the pathopposing the flow of the molten film tube forms compound angles, andwherein a portion of the divergent cooling interface expelling thecooling gas in the path with the flow of the molten film forms compoundangles.
 15. The method according to claim 9, wherein the expelledcooling gas from the at least one divergent cooling element sufficientlycools the molten film tube at a rate between 0.5 and 5(pounds/hour)/(inch of die circumference).
 16. The method according toclaim 10, wherein at least a portion of the cooling gas is received byat least one enclosure comprising a cavity for receiving the cooling gasfrom the at least one divergent cooling element, the at least oneenclosure operable to maintain a predetermined pressure differentialbetween an inside surface and an outside surface of the flow of themolten film tube.